Textile machines, such as weaving machines, knitting machines or the like are fed with yarn, or more generally expressed with thread, from so-called supply bobbins. Certain machines of this type, e.g. most of the modern weaving machines, do not consume the thread at a constant speed, but the thread is drawn intermittently, i.e., by jerks, when the so-called "pick" goes into the machine. "Pick" means the intermittent insertion of the thread, e.g. by means of an air and/or water jet or by means of a mechanical thread guide member.
This intermittent winding off of the thread from the supply bobbin often leads to difficulties in achieving a sufficiently constant and low thread tension, among others due to unfavorable geometry and location of the supply bobbin and also due to the fact that the winding off forces vary as the size of the bobbin decreases when the thread is unwound, that can easily result in thread breakage followed by a standstill of the textile machine. To eliminate these drawbacks and to supply the machine with a constantly checked quantity of thread, a so-called thread feeder or according to the technical terminolgy a "storage feeder" is placed between the supply bobbin and the machine, preferably as close to the thread input of the machine as possible.
In such thread feeders or storage devices, a winding-on-member driven by an electric motor is usually used to wind up the thread onto a stationary storage drum. The thread wraps round on this drum, establishing the desired thread supply, from which the machine pulls off the amount of thread momentarily required.
In order to adapt the speed at which thread is wound onto the drum to the wind-off speed, it is common to control the thread supply on the drum to try to keep this thread supply within certain limits. DE-OS No. 18 09 091 describes a feeding device, which includes an optical sensing device providing for an increase or decrease of the driving motor in case that the thread supply reaches a lower range a or an upper range b, respectively. In a middle range c lying between these two ranges a and b, the driving motor is controlled by a constant control signal, which leads to a certain unvariable speed at which the thread is wound onto the drum. The amount of yarn on the drum within this range c therefore either tends to leave for range a or range b, since in all practical conditions, the take-off speed will never be exactly the same as the speed at which thread is wound onto the drum. If, for example, thread is taken off at a speed which is higher than the speed at which thread is wound onto the drum, the thread supply will leave range c for range a. As soon as the supply reaches range a, the motor speed will be increased and the thread storage will be filled up again. Due to inertia of the system, the thread supply will be filled up to some point within range c and then diminish again. Such variations in the amount of thread supply on the drum and wind-on speeds, however, lead to tension variations in the offgoing thread, which may cause quality variarions in the textile product manufactured on the machine to which such a storage feeder is connected.
The object underlying the present invention is to overcome the above disadvantages and to provide for a method and an apparatus for controlling a thread storage and feeder device, which automatically adapts its winding-on speed to the actual thread consumption.
In a method of the type described above, this object is achieved by generating a control signal for the number of revolutions of the motor which depends both on the amount of the thread supply and the actual thread consumption.
In contrast to the methods used up to now, the method according to the present invention takes into consideration the speed with which the thread is taken off from the storage drum. This results in a method of driving the motor, which is substantially improved compared with known methods, since the consideration of the actual thread consumption during the regulation of the number of revolutions of the motor effects that the number of revolutions is in good correspondence to the thread consumption, so that the thread is wound up continuously. The present invention allows that the thread supply on the intermediate storage is held small, so that the invention meets the above-mentioned requirement to a great extent.
In a preferred embodiment of the present invention, it is provided that the motor is driven with a maximum and a minimum rotational speed if the amount of thread on the thread drum is within a lower range a or an upper range b, and that the rotational speed of the motor is controlled by a control signal derived from the actual thread consumption when the thread supply is within lower limit a so that the thread supply is quickly replenished. On the other hand, the motor speed is sharply decreased or even stopped, if the storage drum is full. Since the thread consumption is taken into account in the regulation, the motor is driven with a rotational speed which corresponds to the thread consumption, when the thread supply lies between said two limits. In the stationary state, the rotational speed of the motor substantially correspond to the thread consumption, i.e., the speed with which the thread is pulled off from the storage drum. In this stage, disadvantageous variations of the thread tension are practically completely eliminated.
In an apparatus for controlling the rotational speed of a winding member of a thread storage and feeding device for thread processing machines which includes a thread drum for momentarily storing thread from a supply bobbin in the form of a thread supply, comprising an electric motor for driving said winding member, sensing means responsive to the amount of said thread supply momentarily present on said thread drum and a signal processing means connected between the sensing means and the electric motor, which in a lower range a, an upper range b and a middle range c, provides for different output signals controlling the rotational speed of the motor, the present invention provides that the signal processing means includes a signal correction means which stores the value of the output signal used in range c before this range has been left for range a or range b and that the signal processing means corrects the value stored in the correction means when the yarn supply is either in range a or range b.
The apparatus according to the invention automatically adapts its winding-on speed to the actual thread consumption by stepwise correction of the control signal for the motor. If the apparatus should have a tendency to leave middle range c for the upper range b, this occurrence is used every time as a criteria that the rotational speed of the motor is still too large and it is decreased stp-by-step until this rotational speed is exactly adapted to the actual thread consumption. The step width may be proportional to or a function of the time periods during which the thread supply stays within either lower range a or upper range b.